1. Field of the Invention
The present invention relates to, for example, cylinders suitable for relatively small two-cycle internal combustion engines which are used for portable machines and the like. In particular, it relates to those optimized in horizontal scavenging angle (collision angle) of fresh gas mixture (air-fuel mixture) blown off from a pair of scavenging ports employing loop scavenging mode to reduce noxious substances in exhaust gas.
2. Description of the Prior Art
Exhaust gas regarded as a principal cause of air pollution is mostly exhausted from boilers used in factories and internal combustion engines mounted on automobiles. Various attempts have been made to take measures for reducing noxious substances contained in such exhaust gas, i.e., NOx (substances resulting from combustion of fuel), CO (substance resulting from incomplete combustion of fuel), HC (substance of unburnt combustible from fuel), and the like.
On the other hand, there has heretofore been taken minor account of relatively small (total displacement is about 35 cc or less) general purpose two-cycle gasoline engines which are used for portable machines and the like because of their small displacement. Recently, however, under increasing tendency to grapple with environmental problems, it is strongly demanded also with respect to such small two-cycle gasoline engines to reduce noxious substances in exhaust gas discharged therefrom.
FIGS. 2 and 3 show one illustrative form of a cylinder of a conventional small air-cooled two-cycle gasoline engine, for which it is urgently needed to reduce noxious substances in exhaust gas, (in FIG. 2, the ports are depicted as if all of them were in the same level for the convenience of explanation).
The cylinder 50 comprises a cylinder portion 2 having a cylindrical inner wall 5 into which a piston is fit-inserted and a head portion 3 formed with a combustion chamber 7 of so-called squish dome type, which are integrally formed with each other, and is formed with a plurality of cooling fins 12 at its peripheral portion. In the cylinder portion 2, an intake port 14 and an exhaust port 16 are so formed as to open oppositely at different levels, and a pair of scavenging ports 20, 20 are oppositely formed which employ Loop scavenging mode (Schneule scavenging mode) symmetrical with respect to the longitudinal section F bisecting the exhaust port 16 (the vertical plane passing along the axis CL of the cylinder 50).
In this form, inner horizontal scavenging angle .alpha. is right angle which is formed by extension lines from inner surfaces 20a, 20a of end potions closer to the exhaust port 16 of the pair of scavenging ports 20, 20, and the intersection Q of the pair of outer horizontal scavenging lines c, d, which are extension lines from inner surfaces 20b, 20b of end portions farther from the exhaust port 16, is arranged to be located on the inner wall surface 5 of the cylinder, and the distance g along the inner wall surface 5 of the cylinder between each of the ends of the inner wall of the exhaust port 16 and the end of the inner surface 20a of each of the scavenging ports 20, 20 is relatively short.
In such a conventional small two-cycle engine using gasoline as fuel as described above, of noxious substances contained in exhaust gas discharged therefrom, there is prospect of reduction of NOx and CO to some extent by optimizing air/fuel ratio or the like. As things stand, however, there have not been found effective measures for reducing HC discharged due to blow-by phenomenon of unburnt air-fuel mixture (fresh gas mixture) inherent in a so-called piston valve type engine.